# Americium-241 is used as the radioactive source in smoke alarms. It is an alpha emitter with a half-life of 432 years. Explain why Americium-241 is well suited for use in a smoke alarm, and explain how a smoke alarm using this source detects smoke.

> OCR A-Level Physics A (H556) — 6.1 Capacitors · Explain · 5 marks

> Americium-241 is an alpha-emitting radioactive isotope with a half-life of 432 years. It is used as the radioactive source inside domestic smoke alarms.

## Mark scheme (5 marks)

1. Alpha radiation is the least penetrating type of radiation / is stopped by a few centimetres of air, so it does not travel far from the source and does not pose a significant radiation hazard to people nearby.
2. The long half-life of 432 years means the source remains active for a very long time and does not need to be replaced.
3. The long half-life means the activity / count rate changes very little over time, so the source gives a steady, reliable output of alpha particles.
4. In normal conditions, alpha particles emitted from the source ionise the air between the detector plates, allowing a small electric current to flow.
5. When smoke enters the alarm, smoke particles absorb / block the alpha particles, reducing ionisation of the air, so the current drops / falls and the alarm sounds.

## Key terms

- [half-life](https://www.gradenine.co.uk/glossary/half-life)

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Source: [GradeNine](https://www.gradenine.co.uk/q/americium-241-is-used-as-the-radioactive-69e395c2) · Published by Druglandscape Ltd.